The present invention has a useful application in screw compressors for supporting the rotors and is particularly advantageous and useful in so called "Dry Air Screw Compressors". In Dry Air Screw Compressors, rotation of the main rotor is transmitted to an auxiliary rotor via synchronizing gears. In these assemblies, the torsional flank clearance of the gear wheels is smaller than that of a screw rotor. These compressors, which operate at high speeds and supply completely oil free compressed air, are typically used in chemical and food processing industries. There is also an increasing demand for oil free air for environmental reasons. Typical bearing mounting arrangements for the rotors of these screw compressors include a thrust bearing arrangement consisting of two single row angular contact ball bearings arranged face to face or back to back, each having the same contact angle. This type of thrust bearing mounting is not new per se and is shown in the prior art patents listed below:
______________________________________ U.S. Pat. No. 4,227,755 Anders Lundberg BEARING ARRANGEMENT FOR SHAFT OF ROTARY COMPRESSOR U.S. Pat. No. 4,730,995 Randy E. Dewhirst SCREW COMPRESSOR BEARING ARRANGEMENT WITH POSITIVE STOP TO ACCOMMODATE THRUST REVERSAL ______________________________________
In these prior assemblies, the contact angle for the duplex set of angular contact ball bearings is in the range of 30.degree. to 40.degree.. One bearing functions to take up axial load which acts in one direction only at steady state operation of the compressor, and the complementary bearing, or back-up bearing, is used to control play in the rotor.
Prior to the present invention, both bearings were designed with the same high contact angle to support the thrust load of the rotors during operation of the compressor.
Prior to the present invention it was believed that providing the thrust bearing and back-up bearing with the same large contact angle would not affect the thrust capability of the bearing system. However, the prior art systems exhibited rather poor life statistics since at high speeds there is an increased internal axial force resulting from centrifugal forces acting on the balls. This means that in these prior art bearing systems the load carrying bearing has to support an axial force greater than the gas force produced by the rotors.
It has been observed that the life of bearings, particularly in demanding applications such as Dry Air Screw Compressors is limited due to the high load and high speed occurring in the system during typical operational. Various attempts have been made to increase life of the bearings in these applications including providing relatively large axial clearances in the bearings as illustrated in FIG. 4. It is noted however, that this solution is not entirely satisfactory since large axial clearances have a negative effect on compressor efficiency. Stated another way, building axial clearance into the bearings impacts on precise axial positioning of the rotors relative to one another and in the compressor housing, and this in turn impacts compressor performance adversely. The larger axial clearances in the bearing (see FIG. 4) necessitates larger clearances between the rotors and housing to avoid so called "rotor rub" and these larger clearances cause leakage of gas from the high pressure chamber or cavity of the compressor to the low pressure cavity. Thus the compressor is less efficient. Furthermore, large axial clearances produce sliding between the balls and the raceways and thus limits bearing life.
There is also disclosed in an MRC publication entitled, "PUMPAC PROGRAM" a bearing arrangement consisting pairs of angular contact ball bearings for various applications. The use of a matched set of bearings wherein one of the bearings has a contact angle of 40.degree. and the other has a contact angle of 15.degree. is disclosed. The PUMPAC design is identified as being for lower speeds wherein the limiting ndm is 550,000. The stated purpose of the matched set configuration in PUMPAC is to eliminate ball sliding and shuttling to produce lower operating temperatures, stable oil viscosity, consistent film thickness and longer service life. It is noted, that PUMPAC is not concerned with reducing internal forces or does not discuss or recognize the effect of centrifugal forces in a bearing system. The benefits of reduced internal loading are not significant in the low speed ranges disclosed in the MRC publication.